As is known, in electro-acoustical transducers for transforming electrical signals into acoustical signals and diffusing the latter, the vibrating member, which is electrically driven by the signal, consists of a diaphragm to the active surface thereof, that is that surface which effects the transmission medium, usually consisting of air, there is rigidly affixed, at the center of mass thereof, a conductor of the acoustical or sound current, i.e. a movable coil which is driven because of the inter-action between the acoustical current and a constant magnetic flux through the gap therein the conductor is arranged.
In general, a typical transducer for a middle-high range is provided with a diaphragm or membrane which essentially consists of an active substantially rigid curved surface, properly designed for the provided frequency spectrum, which is rigid with a supporting frame on which is rigidly affixed the movable coil.
The diaphragm comprises moreover a resilient suspension system, having a suitable resiliency for the frequency range to be reproduced; in other words, this resiliency has a value which, in the parameter system of the diaphragm, for example the provided masses, determines the condition for a linear acoustical reproduction up to the maximum amplitude of the applied signals, with a substantially even response through the overall range; there being moreover provided a peripheral ring member which restrains the movable equipment to the rigid structure of the unit.
More specifically, the rigid surface is provided by a light alloys blade, configured as a spherical cap, or the like, whereas the resilient suspension system comprises a ring member usually made of a thin plastic material, which material also forms the support member of the alluminium wire coil on which it is wound, and the restraining ring member.
For designing such a system, it is necessary to consider several practical requirements imposed by the latest developments in the electric-acoustical field (such as, for example, the great spreading of the recording dynamic range), and the consequent increase of the risk due to the required performance severity imposed to the system from the reliability standpoint.
The most important characteristic for an operating system is the capability of this system to support power, i.e. its capability to preserve, under extreme conditions, the qualitative and quantitative efficiency of less stringent requirements, so as to provide a continuous and even type of operation.
For a maximum power level, among the several chain components, from the starting electrical input signal to the transduced acoustical signal, is the electrical signal to acoustical signal converter which is the most affected component.
In order to bring this reasoning to a very elementary degree, (that is to reduce it, by anarithmetically speaking, to the "last terms"), let us consider a system in which the full acoustic frequency range has been practically divided into several sections, each controlled by a suitable characteristic transducer, and let us consider exclusively the transducer of the top end limit of the range, for example, from 3 k to 16 k Hz, that is that for which the subject reasoning will be clearly evident (naturally under the hypothesis that the considered range is not further subdivided).
For a faithfull reproduction or playback, as stated, apart other requirements which are herein neglected, there is necessary to provide a perfect linearity, that is an even ratio of the value of a parameter, for example the power of the input electrical signal, and that of the corresponding acoustical signal at the output of the transducer; and this for all of the frequencies: which means a substantially even response through the overall frequency range, that is with an offset less than .+-.2 dB.
If the values of the characteristics parameters of the transducer, in particular of those of the movable assembly, have been properly designed in order to provide the desired characteristics, it occurs that the conductor wire used for forming the movable coil will have an insufficient cross-section for supporting the current intensity corresponding to the maximum signals of a recursive average duration, with a consequent superheating; moreover, the diaphgram construction will have an insufficient thickness to resist against the stresses exerted thereon by the movable coil subject to its maximum displacements; on the other hand, its mass can not be increased to allow, considering the mass of the movable coil,a playback of the top limit of the sound frequency range practically devoid of any loss.
By concluding, if the transducer being considered is operated in extreme conditions, as it actually occurs, then the most deleterious drawbacks will be due to: a) the over-heating of the movable coil which is susceptible to deleteriously damage the movable assembly or equipment, because of the high temperature involved; b) the premature wear of the parts of the diaphragm structure, which are subjected to very high mechanical stresses, with a consequent heat generation, such as great stretching and binding deformations, characteristic of the resilient suspension system and mainly because of a very great tension stress to which the diaphgram is subjected along its coupling line to the diaphragm supporting unit, because of instantaneous pulses, even of moderate value, all of which will contribute to quickly put the transducer out of service.
Since it is not possible to increase, in a constant characteristic transducer, the mass of the diaphragm and movable coil, without negatively affecting the result, the single approach to be usedis that of efficiently and quickly dissipating, as fast as possible, the generated heat so as to hold the temperature of the diaphragm, for the provided operation type, under a risk level.
The U.S. Pat. No. 4,843,628 to Hofer discloses an inertial transducer comprising a housing containing therein a magnetic circuit including components thereof separated by a spring diaphragm wherein the flexing of the diaphragm causes the components to move toward and away from each other to induce a current in a coil. The spring diaphragm serves to separate the housing into tuned cavities the frequencies of which differ from each other and from that of the spring diaphragm. This transducer has frequency response peaks at the resonance frequences of the cavities and spring diaphgram.
The Hofer's transducer is not a movable coil transducer but is of the "movable iron" type and such transducer, being of a comparatively low power, does not present particular problems of efficiently dissipating the generated heat.
Thus, Hofer proposes a transducer the diaphragm of which is anchored to a metal mass and resiliently suspended and includes a corrugated portion providing a resilient suspension system, with a pheripheral ring element engaged under a top flat portion of the supporting unit and a central region therefrom a cylindric portion extends which in turn supports a movable electrically conductive coil which closely contacts the metal material of the diaphragm so that, as the transducer is operated the coil transmits to the diaphragm heat generated by the acoustical current passing through the coil in order to dissipate the generated heat: however, in this reference, the coil is not in contact with the metal material of the diaphragm and, on the contrary, from the drawings of this reference it would seem that the coil is insulated from the metal material of the diaphragm by means of a bobbin which is apparently made of an insulating material in order not to short circuit the turns of the coil.
The U.S. Pat. No. 4,709,392 tp Kato relates to modificationsof the diaphragm of an acoustical transducer, such as specifically designed holes therethrough, in order to controllably modify the higher order modes of operation and frequency response of the transducer.
Since also the Kato's transducer is of a comparatively low power, in this reference there is not addressed the problem of efficiently dissipating the generated heat.
The U.S. Pat. No. 4,752,963 to Yainazaky discloses an acoustical transducer in which the coil does not directly contact the diaphragm to transfer the heat generated by the coil to the adjoining metal mass: on the contrary, the peripheral anchoring of the coil is made by using a resilient material which, consequently, will provide a termal cut hindering the conduction of the generated heat to the metal mass of the magnetic circuit.